Rotary Encoder

ABSTRACT

A device for sensing the relative rotary position of first and second parts about a rotation axis, the device comprising a follower constrained to move on a first track fast with the first part and on a second track fast with the second part, the first track being linear and the second track comprising a plurality of circular arcs and at least one transition section connecting one of the circular arcs to another, the tracks being arranged so as to convert relative rotation of the parts into linear motion of the follower, wherein the second track is generally spiral, each circular arc is of constant radius about the rotation axis and the first track is perpendicular to the rotation axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/839,287, filed on Apr. 3, 2020, which is a continuation of U.S.application Ser. No. 15/746,121, now U.S. Pat. No. 10,627,260, filedJan. 19, 2018, which is a National Stage of International ApplicationNo. PCT/GB2016/052263 filed on Jul. 22, 2016, which claims priority toGreat Britain Application No. 1512967.9 filed on Jul. 22, 2015, thecontents of which are hereby incorporated by reference in theirentireties.

This invention relates to rotary encoders, for example for sensingrotary position in robot joints.

Rotary encoders are widely used for sensing the position of rotatableelements such as shafts. Examples of their application include robot armjoints, automotive drive shafts and control wheels or knobs.

One common type of position sensor is the Hall effect magnetic sensor.These sensors have a ring around which is arranged a set of alternatingmagnetic poles. A sensor interacts with the ring, and is located so thatthe magnetic poles move past the sensor as the rotation that is desiredto be sensed takes place. For example, the ring could be attached abouta shaft and the sensor could be attached to a housing within which theshaft rotates. The sensor detects changes in magnetic polarity as thepoles move past the sensor. By counting the number of changes inpolarity the amount of rotation from a reference position can be sensed.To sense the direction of rotation two such pairs of rings and sensorscan be provided, and arranged so that one sensor detects magnetictransitions of its ring at rotation positions that are offset from thepositions where the other sensor detects magnetic transitions of itsring. By considering the relative timing of transitions detected by eachsensor the direction of rotation can be sensed.

Similar properties can be got from other forms of two-state rotationsensing devices, for example optical sensors that sense transitions fromblack to white on a rotating disc, or eddy current sensors that detectthe presence or absence of a tooth on a toothed wheel rotating past asensor.

An enhancement of the approach discussed above is to measure theposition of the poles relative to the sensors to multi-bit accuracy, andto arrange the rings of poles such that each position of the shaftwithin a 360° range is associated with a unique set of outputs from thesensors. This may be achieved by providing different numbers of poles oneach ring and making the numbers of poles the rings co-prime to eachother.

A problem with sensors of this nature is that they can only detectrelative position, or if they can detect absolute position it is onlywithin a range of 360°. The state detected by the sensor(s) isindependent of the number of whole revolutions made by the shaft. Thisis immaterial for certain applications, but for other applications itrequires additional steps to be taken in order to form a measurement ofabsolute position. One example of an application where absolute positionis important is in robotics. Some robot joints may be capable ofrotating more than 360°, and when the robot is operating it is importantto know how many rotations have been undergone from a referenceposition. That information might be necessary to avoid excessivetwisting of internal cables due to driving the joint too far in onedirection, or to provide reassurance that if the robot is reset part-waythrough a procedure any parts the robot was holding when it was resetcan be restored to their original condition. In some applications theshaft whose motion is being sensed is connected to another mechanismthat provides a limit to the shaft's travel after some number ofrotations. In that situation it is common for the encoder to becalibrated by rotating the shaft until the limit is reached and thenresetting the count on the encoder. A count can then be maintained ofthe net number of transitions detected since the shaft was at the limit,the count being incremented or decremented depending on the direction ofrotation. The number of whole rotations undergone since the shaft was atthe limit can be determined by dividing the count by the number oftransitions expected in a full rotation. One problem with this is thatthe shaft must be turned to its limit in order to perform thecalibration. That may be undesirable in some situations, for example ifthe shaft is holding an instrument that is inserted into an object thatcould be damaged by large amounts of rotation of the instrument.

It is desirable to have an improved or alternative way of allowing theposition of a rotating object to be sensed.

According to the present invention there is provided a device forsensing the relative rotary position of first and second parts about arotation axis, the device comprising a follower constrained to move on afirst track fast with the first part and on a second track fast with thesecond part, the first track being linear and the second trackcomprising a plurality of circular arcs and at least one transitionsection connecting one of the circular arcs to another, the tracks beingarranged so as to convert relative rotation of the parts into linearmotion of the follower.

The second track may be generally spiral. Each circular arc may be ofconstant radius about the rotation axis. The first track may beperpendicular to the rotation axis.

Each circular arc may be of a different radius from the other(s).

All the circular arcs may lie in a single plane perpendicular to therotation axis.

The second track may be generally helical. Each circular arc may lie ina single plane perpendicular to the rotation axis. The first track maybe parallel to the rotation axis.

All the circular arcs may be of the same radius.

Each of the circular arcs may lie in a different plane from the other(s)perpendicular to the rotation axis.

Each circular arc may occupy more than 270° of a circle.

The device may further comprise a sensor for sensing the position of thefollower in the first track.

The sensor may be a switch providing a single bit output.

The device may comprise a second sensing mechanism for sensing theabsolute or relative rotary position of the first and second parts aboutthe rotation axis over a range not greater than 360°.

The second sensing mechanism may be a magnetic sensing mechanism.

The first and second tracks may be defined by channels. The follower maybe located in both channels.

The follower may comprise a rigid linear element located in bothchannels.

The second track may comprise a passage open to the radial exterior andcommunicating with the outermost one of the circular arcs whereby thefollower can be introduced into the outermost arc of the second track.

According to a second aspect of the present invention there is provideda robot arm comprising a device as set out above. The device may bearranged for sensing rotation about a joint of the arm. The joint may bea revolute joint arranged so that its rotation axis extendslongitudinally with respect to the limbs of the arm between which it islocated.

The present invention will now be described by way of example withreference to the accompanying drawings.

In the drawings:

FIG. 1 is a general representation of a shaft equipped with a positionencoder mechanism.

FIG. 2 shows a portion of the periphery of a disc 5 of FIG. 1,illustrating a generally spiral track of the position encoder in moredetail.

In the arrangement to be described below, an encoder for the relativerotational position of two objects is capable of absolute determinationof the relative rotational position in whole rotations.

(By “absolute determination” is meant that the position can bedetermined directly from the output of the sensor(s) without, forexample, the need to count up the amount of motion since the relativerotational position was in a reference configuration). The absolutedetermination is preferably over a range greater than 360°. This isachieved by a follower constrained to run (a) in a generally spiral orhelical path about the rotation axis and (b) in a linear path along ortransverse to the rotation axis. The major portions of the spiral orhelical path are circular, so that for the majority of the relativerotational travel of the two objects there is no motion of the followeralong the linear path.

FIG. 1 shows a shaft 1 equipped with a position encoder. The positionencoder is capable of detecting motion of the shaft, the direction ofthat motion and the absolute number of rotations of the shaft from alimit position.

The shaft is configured to rotate about a rotation axis 2. Three discsare attached to the shaft so as to rotate with it. Discs 3 and 4 allowfor magnetic encoding of rotary position. Disc 5 allows for mechanicalencoding of rotary position.

Discs 3 and 4 carry a number of permanent magnets defining magneticpoles 6. On each disc the poles are arranged in a circle having therotation axis 2 of shaft 1 as its axis. On each disc the magnets arearranged so that around the circle of poles the poles exposed at thesensing surfaces 7, 8 alternate between north and south poles. Magneticsensors 9, 10 are disposed adjacent to the sensing surfaces 7, 8 andaligned with the rings of magnetic poles 6. The magnetic sensors arefast with the body relative to which shaft 1 rotates: for example bybeing fixed to a housing for shaft 1. As a result, when the shaftrotates, together with discs 7, 8, the rings of magnetic poles 6 revolvepast the sensors 9, 10. The sensors are capable of detecting transitionsbetween north and south poles in the ring of poles as such transitionsmove past the sensors. The sensors could, for example be Hall effectsensors, reed sensors, magnetoresistive sensors or inductive sensors.For relative position sensing each sensor 9, 10 is arranged so that whena transition from a north pole to a south pole passes the sensor theoutput of the sensor goes from high to low, and when a transition from asouth pole to a north pole passes the sensor the output of the sensorgoes from low to high. For absolute position sensing within a range of360° each sensor is arranged to provide a multi-bit output representingthe relative position of the neighbouring poles to it and the rings ofpoles are arranged such that each position of the shaft within a 360°range is associated with a unique set of outputs from the sensors. Thismay be achieved by providing different numbers of poles on each ring andmaking the numbers of poles the rings co-prime to each other. Theoutputs from the sensors pass to a processing unit 11.

The circumferential positions of the sensors 9, 10 and the rotationalpositions of the disc 7, 8 about axis 2 are chosen so that thetransitions between the poles on disc 7 as sensed by sensor 9 occur fordifferent rotational positions of the shaft from the transitions betweenthe poles on disc 8 as sensed by sensor 10. This allows the direction ofrotation of the shaft to be inferred from the relative order of high/lowand low/high transitions as sensed by each sensor. The rings and sensorsallow a relative position of the shaft to be determined.

The number of magnetic poles around the discs can be selected based onthe application; but there could, for example, be around 30 to 40 pairsof north/south poles. For absolute position sensing within a 360° rangeusing the technique described above the numbers of pairs on the ringsshould be co-prime.

The mechanical encoder comprising disc 5 will now be described.

Disc 5 is fast with the shaft 1 so as to rotate with the shaft. Disc 5has a series of formations 20 which have an extent along the directionof the shaft. The formations define a generally spiral path 21 in theplane of the disc 5, i.e. in a plane perpendicular to the axis of theshaft. A follower 30 is guided by the formations in a radial direction(i.e. in a direction perpendicular to the axis of the shaft), so thatrotation of the shaft can cause motion of the follower in a radialdirection. As will be described below, the radial position of thefollower can be detected in order to establish the absolute position ofthe shaft 1 in whole rotations from a reference position.

In more detail, disc 5 has a formation of ridges 20 which extend in anaxial direction: i.e. along the axis 2 of the shaft 1. The ridges areconfigured so as to define a generally spiral groove 21 between them.For the majority of its path the spiral groove is of constant radiusabout the rotation axis 2, for example as indicated in FIGS. 2 at 22 and23. The region 22 is of a first radius, and the region 23, which iswithin region 22, is of a second radius smaller than the first radius.In a transition region 24 the ridges are configured so as to define asmooth change in radius of the groove between the first radius and thesecond radius, as indicated at 25. The interior track 23 of the grooveterminates in an end wall 26. The exterior track 23 of the grooveterminates in a radial passageway 27 which extends radially outwardlyand opens to the circumference of the disc 5.

A mechanical sensing device 28 is located adjacent to disc 5. Themechanical sensing device is fast with the body relative to which shaft1 rotates: for example by being fixed to a housing for shaft 1. As aresult, when the shaft rotates the disc 5, which is fast with the shaft,rotates past the sensing device 28. The sensing device has a track 29within which follower 30 is constrained to move. The track 29 isdirected perpendicular to axis 2. The follower is a pin extending alongthe axis 2. The pin runs snugly in the groove 21 and also in the track29. The mechanical sensing device is arranged to maintain theorientation of the pin parallel to the axis 2, for example byconstraining a flat head 31 of the pin (shown with a D-shape in FIG. 2)to prevent rotation of the head about axes perpendicular to axis 2.

The interaction of the track 29, the groove 21 and the follower 30 issuch that when the disc 5 is rotated the position of the follower in aradial direction within the groove is controlled by its running ingroove 21. When one of the regions of the groove 22, 23 that haveconstant cross-section are aligned with the track 29 the disc can rotatewithout the follower 30 moving in track 29. The follower will remain ineither a radially outward position aligned with groove portion 22 or aradially inward position aligned with groove portion 23. When the shaftis rotated so that the transition region 24 revolves past the track 29the follower is forced from a radially inward position to a radiallyoutward position (when the shaft is turned anti-clockwise as viewed inFIG. 2) or from a radially outward position to a radially inwardposition (when the shaft is turned clockwise as viewed in FIG. 2).

A microswitch 32 is positioned so as to detect when the follower 30 isin its radially outward position: i.e. aligned with groove portion 22.The output of the microswitch forms the output of the mechanical sensordevice 28, and is passed to the processing unit 11. The radial positionof the follower 30 in track 29 could be detected in other ways. Forexample, its presence could be detected at the radially inner positionrather than the radially outer position; and the detector could be asingle bit (on/off) switch (e.g. a mechanical, magnetic or opticalswitch) or could provide a more detailed indication of position alongthe length of the track 29. The output of the sensor 28 is an absoluteposition signal indicating the number of revolutions of the shaft from areference point.

The outputs of sensors 9, 10, 28 pass to the processing unit 11. Theprocessing unit comprises a processor device 40, which could be hardcoded to interpret the signals from the sensors 9, 10, 28 or could be ageneral purpose processor configured to execute software code stored ina non-transient way in memory 41. The processor device combines thesignals from the sensors to form an integrated output signal at 42.

The data from the sensors may be used by the processor device 40 in anumber of ways.

1. The mechanical encoder arrangement 5, 28 could be implemented (eitherby itself or together with another position encoder such as thatprovided by discs and sensors 3, 4, 9, 10) to provide a simple outputrepresenting the number of whole revolutions of the shaft 1 from areference location. If the reference location is taken to be the end 26of the path 21 then non-detection of the follower 30 by micro switch 32could indicate zero revolutions and detection of the follower 30 bymicroswitch 32 could indicate one revolution.

2. The mechanical encoder arrangement 5, 28 could be used to indicate areference location for resetting the relative position count associatedwith the relative position measurement system. When the count is desiredto be reset the output of the mechanical position encoder is known. Theshaft 1 is then rotated in a direction selected in dependence on thatoutput so as to move the transition zone 24 towards the follower. Withreference to FIG. 2, if the output of the mechanical position encoderindicates that the follower is in outer track region 22 then the shaftis rotated clockwise and if the output of the mechanical positionencoder indicates that the follower is in inner track region 23 then theshaft is rotated anti-clockwise. This determination may be made by theprocessing unit 11 in response to a signal to reset the count, andsignalled to a drive unit (e.g. a motor) for driving the shaft in theappropriate direction. When the processing unit subsequently detects atransition of the output of the mechanical position encoder it knowsthat the transition zone is aligned with the sensor 28. At that pointthe count can be reset. This approach has the advantage that it avoidsthe need to move the shaft to an extreme position to reset the count ata position where both the rotational position and the number ofrotations of the shaft from a predetermined reference position areknown. It may be that the precise rotational position of the shaft atwhich the output of the mechanical switch transitions when the followeris in the transition region 28 is different depending on the rotationdirection of the shaft. In that case, the same procedure as above can befollowed, but in one direction of the shaft when the transition isdetected in the output from sensor 28 the shaft is then turned in theopposite direction and the subsequent transition used to indicateresetting of the count.

3. In the example described above, the discs and sensors 7, 8, 9, 10 arecapable of sensing relative position. In an alternative arrangement theycould be capable of sensing absolute position within a 360° revolutionof the shaft. This can be done in a number of ways. For example, thesensors 9, 10 could be capable of sensing their relative positionbetween magnetic poles 6 and outputting an analogue or multi-bitrepresentation of that relative position, and the numbers of poles onthe discs 7, 8 could be selected so that in combination the sensors 9,10 yield a value uniquely representing the position of the shaft withina 360° revolution. In another example, the poles could be located on thediscs in a binary encoded fashion, so that in combination the sensorsyield a digital output uniquely representing the position of the shaftwithin a 360° revolution. In combination with the mechanical encodingarrangement 5, 24 this approach allows the absolute position of theshaft both within a 360° revolution and in whole revolutions from an endpoint to be immediately determined without the need for movement of theshaft. This is useful in that it allows the position of the shaft to befully determined immediately on start-up of the system, without therequirement for a calibration step as discussed at 2 above.

In the example discussed above, the groove 21 varies in radius aboutaxis 2 and the radial position of the follower 30 indicates the absoluteposition of the shaft. In an alternative arrangement the grove could bea generally helical groove, and the follower could move axially in thegroove to indicate absolute shaft position. Over the majority of itslength such a helical groove would be of constant position along axis 2,and there would be a transition zone in which its axial position varieswith radial position of the shaft. The track 29 would be disposed in anaxial direction. The sensor 32 would sense axial motion of the follower.

The part circular arcs 22, 23 or their helical equivalents preferablyextend over more than half a circle, more preferably over more than270°, more preferably over more than 300°. The transition zone 25preferably extends at 50°or less to the arcs where it connects to them,as shown in FIG. 2. The transition zone preferably includes a linearpath section.

Instead of running in a groove 21/29 the follower 30 could be guided inanother way, for example by riding on a raised track which it overlapson either side.

Slot 27 which extends to the periphery of the disc 5 can be used to helpassemble the mechanical position encoder. In the example of FIGS. 1 and2, during assembly the sensor 28 can be introduced radially to the discand the follower inserted into the spiral channel 21 through the slot27.

In the example of FIGS. 1 and 2 the mechanical sensor can distinguishonly between zero and one whole rotations from the end stop 26. Thespiral groove could be extended to cover more revolutions, with a zoneof constant radius for each rotation covered by the groove and atransition zone between each pair of adjacent constant radius zones. Thetransition zones would be located at a common circumferential positionabout the rotation axis 2. Similarly, in the case of a helical groovethe groove could be extended to cover more revolutions, with a zone ofconstant axial position for each rotation covered by the groove and atransition zone between each pair of adjacent constant axial positionzones. In this case the transition zones would again be located at acommon circumferential position about the rotation axis 2. The sensordevice 28 would be adapted so that it can determine which of theconstant radial/axial position zones the follower is aligned with. Forexample, the sensor device could have multiple microswitches, onealigned with each of the constant radial/axial position zones, or withall but one of those zones.

In the example above, the discs and sensors 3, 4, 9, 10 sense relativeposition by means of magnetic interaction between the discs and thesensors. They could sense motion in other ways. For example the sensorscould be optical sensors that sense transitions from one colour orreflectivity to another on a rotating disc, or the sensors could be eddycurrent or other electrical sensors that detect the presence or absenceof a tooth on a toothed wheel rotating past a sensor. If the sensors 3,4 are magnetic sensors they could be of any suitable type, for exampleHall effect or reed switch sensors.

The control mechanism for the shaft could be arranged so as toautomatically prevent over-rotation of the shaft past the end positions26, 27 in dependence on position data output by the processing unit 11.

Instead of being attached to a shaft the discs could be attached to anyother part that rotates relative to another part. In the case of ashaft, one or more of the discs could be attached to the housing of theshaft and the sensors could rotate with the shaft. The discs could bereplaced by members having the same function but different shapes, e.g.they could be of the form of a cylinder, an annulus or a cuboid.

Some applications of the arrangement are as follows. The arrangementcould be used in a robot arm in which rigid members forming the limbs ofthe arm are coupled by revolute joints. The shaft could be fast with onelimb of the arm and the housing could be formed by the adjoining limb ofthe arm, revolution of the shaft relative to that housing representingrelative rotation of the two limbs. The position encoding arrangementcan then be used to establish the relative position of the arms. Inanother example, the shaft could be the shaft extending from a vehicle'ssteering wheel and the housing could be the housing for that shaft,which is part of the main body of the vehicle. Motion of the steeringshaft could be sensed to control an electrical power steering system orsimply to establish the steering demand (e.g. so as to help control thevehicle's stability control system).

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. A method of calibrating a device for sensing the relative rotaryposition of first and second parts about a rotation axis, the devicecomprising a mechanical encoder arrangement and a relative positionmeasurement system, the method comprising: receiving an output of themechanical encoder arrangement at a processing unit, the mechanicalencoder arrangement comprising a follower constrained to move on a firsttrack fast with the first part and on a second track fast with thesecond part, the first track being linear and the second trackcomprising a plurality of circular arcs and at least one transitionsection connecting one of the circular arcs to another, the tracks beingarranged so as to convert relative rotation of the parts into linearmotion of the follower, and a first sensor for detecting the position ofthe follower in the first track; the processing unit causing the firstpart to rotate in a direction selected in dependence on the output ofthe mechanical encoder arrangement so as to move the at least onetransition section towards the follower; and in response to detecting,at the processing unit, a transition of the output of the mechanicalencoder arrangement, resetting a count of a number of transitionsdetected by the relative position measurement system, the relativeposition measurement system comprising a disc or wheel fast with thefirst part and a second sensor fast with the second part, the secondsensor configured to detect transitions in the disc or wheel when thedisc or wheel moves past the sensor.
 2. The method of claim 1, whereinthe output of the mechanical encoder arrangement indicates which of theplurality of circular arcs the follower is in.
 3. The method of claim 2,wherein the processing unit causes the first part to rotate in a firstdirection when the output of the mechanical encoder arrangementindicates that the follower is in a first circular arc of the pluralityof circular arcs, and the processing unit causes the first part torotate in a second direction when the output of the mechanical encoderarrangement indicates that the follower is in a second circular arc ofthe plurality of circular arcs.
 4. The method of claim 3, wherein one ofthe first and second directions is clockwise and the other of the firstand second directions is anti-clockwise.
 5. The method of claim 3,further comprising: when the selected direction is a specific one of thefirst and second directions, in response to detecting a first transitionof the output of the mechanical encoder arrangement, causing the firstpart to rotate in the other of the first and second directions; andwherein the resetting of the counter is performed in response todetecting a second transition of the output of the mechanical encoderarrangement.
 6. The method of claim 1, wherein the first sensor is aswitch providing a single bit output.
 7. The method of claim 1, whereinthe count is incremented or decremented when the second sensor detects atransition.
 8. The method of claim 1, wherein the disc or wheelcomprises a ring around which is arranged a set of alternating magneticpoles, and the second sensor is a magnetic sensor configured to detecttransitions in magnetic polarity when the poles moves past the secondsensor.
 9. The method of claim 1, wherein causing the first part torotate in the selected direction comprises signalling to a drive unitfor driving the first part to drive the first part to rotate in theselected direction.
 10. The method of claim 1, wherein the causing thefirst part to rotate in the selected direction is performed in responseto receiving a signal to reset the count.
 11. The method of claim 1,wherein the second track is generally spiral, each circular arc is ofconstant radius about the rotation axis and the first track isperpendicular to the rotation axis.
 12. The method of claim 11, whereineach circular arc is of a different radius from the other(s), and/orwherein all the circular arcs lie in a single plane perpendicular to therotation axis.
 13. The method of claim 1, wherein the second track isgenerally helical, each circular arc lies in a single planeperpendicular to the rotation axis and the first track is parallel tothe rotation axis.
 14. The method of claim 13, wherein all the circulararcs are of the same radius, and/or wherein each of the circular arcslies in a different plane from the other(s) perpendicular to therotation axis.
 15. The method of claim 1, wherein each circular arcoccupies more than 270° of a circle.
 16. The method of claim 1, whereinthe first and second tracks are defined by channels and the follower islocated in both channels.
 17. The method of claim 16, wherein thefollower comprises a rigid linear element located in both channels. 18.The method of claim 16, wherein the second track is generally spiral,each circular arc is of constant radius about the rotation axis and thefirst track is perpendicular to the rotation axis, and wherein thesecond track comprises a passage open to the radial exterior andcommunicating with the outermost one of the circular arcs whereby thefollower can be introduced into the outermost arc of the second track.19. The method of claim 1, wherein the device is arranged for sensingrotation about a joint of a robot arm.
 20. The method of claim 19,wherein the joint is a revolute joint arranged so that its rotation axisextends longitudinally with respect to the limbs of the arm betweenwhich it is located.